We’ve talked elsewhere about the danger of extreme heat for concrete, and rounded off with a quick and effective solution to avoid thermal cracking that comes about in thick concrete structures, shrinkage cracking in hot or windy conditions, and early chloride ingress and other problems associated with marine environments.
We’d thought we’d jump on Winter issues as well.
You see, there’s a misconception on many construction sites that concrete does not need to be cured in the cold, because less heat means less evaporation.
This is not the truth.
For two reasons…
1. Concrete takes longer to reach sufficient strength in ambient temperatures under 10°C.
That’s pretty much all that needs to be said about that. It’s not a very low temperature, compared to many Winter averages around Australia and New Zealand – so many construction sites that do early morning pours are likely to be affected by this.
If you want to ensure early strength gains in the concrete slab so that it can be loaded sooner, then the concrete needs some sort of heat. Not necessarily extra heat, but we’ll get to that…
2. Great temperature differentials between the surface of concrete slabs and the inside (or centre, core, inner) cause Thermal Cracking.
This is often seen in thick concrete pours, such as bridge headstocks and wind turbine bases. How it happens is the centre of the concrete heats up dramatically, due to concrete’s own exothermic heat of hydration, while the surface remains cool. Consequently, the surface contracts while the inner does not, and cracks. [1]
This is similar to Plastic Shrinkage Cracking, where the surface has not gained enough strength and is still too plastic to retain tension across the surface, and tears apart. (However, Plastic Shrinkage is due to loss of moisture such as by evaporation. [2])
And… the issue is compounded in Winter due to extra cool concrete surface temperatures.
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Did you notice the startling solution jumping out?
- Concrete needs heat to cure sufficiently
- Concrete creates its own heat – maybe even too much in one place
The heat from hydration rises from lower down, and dissipates at the cool surface.
Concrete can solve its own problem, to keep the surface warm while curing!
The heat needs to be kept in, though. This is where Thermal Curing Blankets come in again.
Covering the concrete properly with thermal curing blankets (TCBs) means the heat will not be lost so quickly – it will be distributed evenly throughout the concrete, which does two things: (1) Allows the concrete at the top to cure and gain strength sufficiently, and (2) Helps the outer and inner sections to cool and contract at the same rate.
Have a look at some TCBs in action around AU and NZ!
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Definitely get in touch if you are interested in preventing cold weather concrete problems with this!
Heat also causes DEF
When we mentioned “Concrete creates its own heat – maybe even too much in one place”, that was a reference to DEF (Delayed Ettringite Formation). This is another issue that occurs in mass concrete and steam-cured concrete when the concrete sets and the cement has not hydrated properly due to high temperatures.
Moisture entering the concrete matrix later on starts the curing process for unhydrated cement, and causes expansive cracking. (Ettringite is simply a crystallite mineral that forms in the early stages of hydration.) [3]
This is a problem that MARKHAM solves with colloidal silica admixtures (liquid, not crystalline) that increase impermeability deep down, so that moisture cannot pass through the concrete and reignite the curing process.
But that’s for another day…
References:
1. Thermal Cracking of Concrete – NRMCA
2. What is Shrinkage Cracking in Concrete? – The Constructor
3. Delayed Ettringite Formation – Science Direct
